The condition you are experiencing is very normal and would be common to all guitar/bass/piano or any other amps using tubes throughout.

The input jack of the amplifier has a safety switch on it that turns the input off when nothing is plugged in. This is to keep the tubes from running away into feedback.

Tubes are different from transistors in that they are always on - always ready to “amplify” any signal they may encounter. A transistor is always off, and waits for a sufficient signal (the opposite of a tube)

If something in plugged into the amp, the safety switch is defeated, the circuit is ON and ready for anything that wants to come in.

A guitar is easily excited into producing a signal, just walking around will cause the strings to vibrate a bit and send a signal to the amp.

The amp will amplify this small signal causing the speaker to move some air. (That’s how speakers work) the nearby guitar’ strings are vibrated by the air from the speaker creating a stronger signal, which is amplified again producing, again, the same result with the guitar - eventually the amp gets real loud and runs away - the guitar strings are really vibrating now and the amp gets too loud - it squeals or “feeds back.”

In any case, an amp should not be left on with something plugged in. It may run away. This is normal operating conditions for an amplifier and is mentioned in the amp’s instruction books from the 60's.

Put the amp on standby if you are going to leave something plugged in, that will keep this from happening.

The ability to create this kind of feedback, under control, is the characteristic of guitar amp combinations that created modern rock.

Long sustaining notes common to Eric Clapton, Carlos Santana, Jimmy Page, Jeff Beck, Eddie Van Halen, Joe Satriani and hundreds of others through the years are the result of practicing with a guitar and amp so this kind of “guitar excitement” feedback can be use for sustain of a single note.

This kind of feedback is far more common with open back combo amps than closed cabinets (Showman, Bandmaster etc.) And is the reason combo amps are still popular and still manufactured.